Construction machines having holding regions, in particular machines for changing a surface with at least one bucket, preferably excavators, in particular heavy cable-operated excavators with large-volume buckets, are used for excavating or removing material. For removing material in open cast mining, for example, bucket wheel excavators which can hold up to 100 metric tons of material in a bucket are used. The excavators can be divided into excavators having an at least two-part arm and buckets arranged displaceably thereon and into cable-operated excavators having at least one boom. In the case of the cable-operated excavators, there are both those in which the bucket hangs from a first cable and is dragged by a second cable and those in which the bucket is fastened to an arm and the cable operates the arm with the bucket. In the case of the excavators having two-part arms, there are those having buckets open at the back and those having buckets open at the front.
In order to avoid overloading when loading giant trucks but nevertheless to achieve as high a load as possible, weight determinations are carried out by the excavators during the filling. Moreover, a measurement of the weight permits calculation of the mass removed.
U.S. Pat. No. 6,225,574 disclose that weight determinations are possible by determining the motor power of bucket drives. These weight determinations are very inexact because they deliver only a total force which is composed of that fraction of the weight to be determined which varies with the bucket movement and of inertial forces which vary with the complicated dynamic movement processes. More exact weight determinations have therefore been proposed. By simultaneous measurement of positions of the excavator structure and loads during a plurality of movement intervals, a loading weight is to be determined with greater accuracy by selection and averaging. By means of the position-measuring series, geometrical and dynamic corrections can be made. At least two sensors must be provided just for an exact determination of the position of the bucket of a cable-operated excavator having a tiltable boom and an arm tiltably fastened thereto and carrying the bucket. Because the excavator is optionally also used on sloping terrain, the determination of the bucket position relative to the pivot joint is more complicated.
The bucket load is determined via the motor power at a steel cable, the steel cable being led from the motor over a pulley to the bucket. In order to be able to derive as accurate a weight component of the bucket load as possible from the measured cable load, the absolute position of the pulley for the steel cable must be taken into account in relation to the absolute position of the bucket, or the orientation of the terrain and the orientation of the boom and of the arm with the bucket. Moreover, the bucket speed and bucket acceleration must also be taken into account.
For a solution according to U.S. Pat. No. 6,225,574, various sensors distributed around the excavator, and a central control, are required. The setup of the measuring system is very complicated and susceptible to faults. In addition, a calibration procedure has to be carried out before operation and selection and averaging steps during operation. A complicated fuzzy logic formulation is required. The operation of the measuring system is complicated. Because the motor power is dependent not only on the bucket load and the instantaneous position and movement of the excavator components but also on the state of the bearings of the moving parts, the accuracy of measurement is also impaired by further parameters which cannot be measured.
The prior art also discloses solutions in which vibrations generated by the load pick-up are measured and weight values are derived therefrom. These measurements are based on the fact that an arm or a boom can be considered as a vibrating system whose vibrations depend on the bucket load. The weight values determined are frequently not sufficiently accurate.
Measuring systems of the loaded truck can also be used for the weight determination but are frequently very inaccurate. Weighers on which the weights of the trucks with and without a load can be determined are also used for the weight determination. The disadvantage of the weighers is that the trucks have to be driven onto the weigher and that an excessive load is not detected until after loading or the truck would have to be continuously monitored during the loading with regard to the load weight, for example in that the loading takes place with a truck standing on a weigher, which is often complicated or unfeasible.
A further problem is the alteration of physical properties, e.g. density or humidity, during production or movement of the material to be loaded. One of the goals is to quantify production—volumes of dirt, ore or soil moved in “bank cubic meters (BCM)”, i.e. in situ volume prior to initial blasting. Especially after blasting and after loading into a bucket a “swelling” of the material occurs, so the physical properties of the dirt in a bucked are different to those in situ prior to blasting or processing. The existing BCM volume estimates based on weights alone can be less accurate due to varying properties, especially a varying density.